1. Field of the Invention
The present invention relates to an electrophotographic apparatus such as a laser beam printer and the like.
2. Related Background Art
FIG. 4 is a schematic constructural view showing a conventional image forming apparatus.
In such an image forming apparatus, as shown in FIG. 4, a photosensitive drum (latent image bearing member) 101 constituted by coating a photo-conductive layer on a cylindrical conductive substrate is supported for rotation in a direction shown by the arrow R1. Around the photosensitive drum 101, there are disposed, in order along a rotational direction thereof, a scorotron charger 102 for uniformly charging a surface of the photosensitive drum 101, an exposure device 140 for reading an original and for forming an electrostatic latent image by exposing the photosensitive drum 101 in accordance with an image signal, a developing device 104 for forming a toner image by adhering toner to the electrostatic latent image, a surface potential sensor (potential detecting means) 141 for detecting surface potential of the photosensitive drum 101, a corona transfer charger (transfer charger) 108 for transferring the toner image formed on the photosensitive drum 101 onto a transfer sheet (recording material) P, an electrostatic separating charger 109 for separating the transfer sheet P to which the toner image was transferred from the photosensitive drum 101, a cleaning device 113 for removing residual toner from the photosensitive drum 101 after the toner image transferring, and pre-exposure means 130 for eliminating residual charge on the photosensitive drum 101.
After separated from the photosensitive drum 101, the transfer sheet P to which the toner image was transferred by the transfer charger 108 is conveyed to a fixing device 112, where the toner image is fixed to the transfer sheet to obtain a desired print image, and the transfer sheet P to which the toner image was fixed is discharged out of a main body of the image forming apparatus.
In an image scanner portion 118, an original 115 rested on an original glass stand 114 is scanned and read by an illumination lamp 116, and light reflected from the original scanned by the illumination lamp 116 is focused on a photo-electric converting element 119 is through mirrors 117a, 117b, 117c and a lens 117d. The electrical signal outputted from the photo-electric converting element 119 is A/D-converted by an A/D-converter 121 into a digital signal which is in turn converted by control means 122 into an image signal proportional to image density. The image signal is sent to a laser driver 124 for generating a signal for driving a laser 120, thereby modulating light emission of the laser (light emitting means) 120 in accordance with the image signal. A laser beam 103 modulated in accordance with the image signal is sent to the photosensitive drum 101 as image information through a polygon mirror 128 and a mirror 117e, thereby forming the electrostatic latent image.
Due to dispersion in manufacturing accuracy, there are photosensitive drums having good sensitivity and photosensitive drums having bad sensitivity. Further, the sensitivity of the photosensitive drum is changed by change in a sensitive property of the photosensitive drum influenced by endurance and/or a surrounding environmental condition.
In order to absorb or eliminate such dispersion, there has been a technique in which the surface potential sensor 141 contacted with the surface of the photosensitive drum is provided within the image forming apparatus, so that a light emitting amount of the laser 120 is adjusted by the control means 122 to keep the surface potential of the photosensitive drum to desired potential, on the basis of the fact that, as shown in FIG. 5, the surface potential of the photosensitive drum depends upon the light emitting amount of the laser.
Among methods for controlling the surface potential of the photosensitive drum, there is a method in which, for example, exposure control coefficient xcex2 associated with a relationship between the light emitting amount of the laser and the surface potential Vd of the photosensitive drum, as shown by the following equation is repeatedly used:
LP(i)=LP(ixe2x88x921)+xcex2(Vdtxe2x88x92Vd(ixe2x88x921)) 
i=1, 2, 3, . . .
Where, Vdt is a target value of potential of dark portion, LP(i) is light emitting amount of laser after i-th correction, Vd(i) is potential of dark portion after i-th correction, xcex2 is exposure control coefficient, and LP(0) is initial value of light emitting amount of laser.
On the other hand, in order to improve an image forming speed and a resolving power, there was a technique in which a plurality of lasers are used for image formation. In such a case, the above-mentioned potential control was repeatedly effected for each laser.
However, in such a conventional image forming apparatus, it took a long time to obtain the desired surface potential of the photosensitive drum or the desired surface potential of the photosensitive drum could not be obtained, or there might occur poor image formation due to slight difference in light amounts of lasers.
One embodiment of the present invention is directed to an electrophotographic apparatus. The apparatus includes a photosensitive member, a first light emitting unit and second light emitting unit. The first and second light emitting units expose the photosensitive member charged, in accordance with image information. The second light emitting unit exposes a second portion of the photosensitive member that is different from a first portion of the photosensitive member, which is exposed by the first light emitting unit. A detecting unit is adapted to detect a potential of an area on the photosensitive member and detects a potential of the first portion of the photosensitive member and a potential of the second portion of the photosensitive member simultaneously. A control unit is adapted to control amounts of light emitted from the first and second light emitting units in accordance with a detection result of the detecting unit.
An object of the present invention is to provide an electrophotographic apparatus in which light emitting amounts of a plurality of light emitting means can be adjusted for a short time.
Another object of the present invention is to provide an electrophotographic apparatus in which, even when a plurality of light emitting means are used, potential control of a surface of a photosensitive member can be effected for a short time, regardless of dispersion in sensitivity (with respect to illumination light) of the photosensitive drum due to change in endurance and/or change in a surrounding environmental condition.
The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings.